1. Field of the Invention
The present invention relates to a hydraulic drive type partial inter-tube lancing system for cleaning a steam generator in a nuclear power plant, which can remove foreign matter, such as sludge, deposited around heat transfer tubes in the steam generator, and more particularly, to a hydraulic drive type partial inter-tube lancing system, which serves to clean a steam generator of a light-water reactor in a nuclear power plant by removing sludge, attached inside the steam generator, using high-pressure water.
2. Description of the Related Art
Generally, a steam generator provided in a nuclear power plant is a very important structure, wherein primary cooling water, to which heat energy generated from a reactor is transferred, and secondary cooling water meet each other and undergo heat exchange on the boundary of heat transfer tubes in the steam generator.
As a turbine is operated by steam, generated via the above-described heat exchange in the steam generator, to convert kinetic energy into electric energy, generation of electricity is accomplished. However, as the operating history of the nuclear power plant increases, wear of secondary system tubes occurs and thus, metal components separated from the tubes are deposited, as sludge, in the steam generator.
The sludge in the steam generator is divided into relatively soft sludge and relatively hard sludge, in view of the strength thereof. In the case of soft sludge, it is mainly deposited on an upper layer tube sheet in the steam generator. On the other hand, sludge deposited on a lower layer tube sheet in the steam generator becomes hard by heat of heat transfer tubes.
The deposited sludge deteriorates heat-transfer ability of small-diameter tubes in the steam generator, resulting in a reduction in heat output efficiency. This is directly connected with a reduction in the output of the nuclear power plant. Stoppage of a nuclear power plant due to deterioration in output has been reported in some countries.
Furthermore, local corrosion of the steam generator causes premature exchange of the steam generator, resulting in enormous economic loss. Therefore, conventionally, a nuclear power plant periodically undergoes a planned precautionary maintenance service and in particular, during the planned precautionary maintenance service, removal of sludge deposited on the upper end of the tube sheet in the steam generator is attempted.
Here, methods for removing the sludge in the steam generator include a high-pressure water ejection method, a chemical cleaning method, an ultrasonic cleaning method, a complex cleaning method and the like. Of these various methods, the chemical cleaning method exhibits the most outstanding cleaning efficiency, but has a weak point, for example, corrosion of small-diameter tubes and therefore, implementation of the chemical cleaning method has been conventionally limited to only two or three times during the entire lifespan of a nuclear power plant.
In the case of the recently developed ultrasonic cleaning method, development of various related cleaning technologies is actively in progress, but accurate evaluation of cleaning efficiency has yet to be presented.
In a state wherein a distinguished alternative method is not yet developed, accordingly, it can be said that the high-pressure water ejection method is an actual cleaning method as one of most advantageous proposals.
One example of a conventional apparatus for removing sludge deposited in a steam generator is disclosed, by the applicant of the present invention, in Korean Registered Patent No. 575,110 (also registered as U.S. Pat. No. 7,086,353). However, the disclosed conventional technology has many problems, as follows. First, due to a bulky volume of a rail, a lance, which is adapted to enter between heat transfer tubes in a steam generator, must be inevitably reduced in length. Further, the lance performs cleaning of only one row of heat transfer tubes via one ejection of high-pressure water, requiring an excessively long cleaning time. As a conclusive problem, drive power, required to fold and unfold the lance or to perform twisting rotation of the lance, is supplied by a servo motor. The servo motor is exposed to high radiation energy in the steam generator and therefore, suffers from frequent failures due to deterioration thereof by the high radiation energy.